Perennials continue to be an increasingly popular trend at the retail level, but perennial production poses different challenges than annual production, especially for inexperienced growers. To succeed with perennials, growers should understand their markets as well as the growing requirements and flowering triggers for the specific perennial varieties they are growing. The primary factors that influence flowering in perennials are photoperiod (day length) and vernalization (cooling) requirements.
Many retailers use blooming perennials to generate impulse sales and extend the sales season. Marketing efforts can easily tie perennials to gardening trends, including hummingbird and butterfly gardening, shade gardening, native plant gardening, xeriscaping, water gardening and fragrance gardens. Perennials also can be used to good effect in mixed containers, whether combined with annuals or other perennials.
Perennials have a range of different flowering triggers, and specific triggers will often vary greatly among cultivars within a species. In general, a perennial can have a photoperiod (day length) requirement, vernalization (wintering over) requirement or a combination of both. Understanding the specific flowering triggers for perennials is one of the most critical factors affecting success for growers.
Photoperiod. Perennials often have a photoperiod requirement for flowering. Long-day perennials require 14 hours or more of light per day to flower. Day-neutral perennials will flower regardless of day length, though in some species and cultivars flowering may be accelerated under long days. Finally, short-day perennials require short photoperiods (generally under 12 to 14 hours of light) to flower.
Long-day perennials can be brought into bloom outside their natural blooming season by using artificial lighting. An easy strategy for providing artificial long days is night-break lighting — providing four hours of light from 10 p.m to 2 a.m. (It is actually short nights that trigger flowering in long-day perennials.) It takes just 5-10 foot-candles of incandescent light for night-break lighting. Day extension with HID lights can also be used to provide long days, at higher energy expense. Stretching can occur when artificial long days are used to flower perennials, especially with incandescent light. To minimize stretch, turn off lights once flower buds are visible.
Some long-day perennials (notably Digitalis purpurea and some gaillardia cultivars) are light accumulators, in addition to having a long-day requirement. Much like zonal geraniums, these perennials flower more quickly when grown under high light levels. Simple night-break lighting may not work efficiently for forcing them into bloom, resulting in non-uniform flowering and low percentage of blooming. Day extension with HID light is a better option than night-break lighting for light accumulating perennials.
Vernalization. A number of perennial suppliers offer vernalized perennial liners. There are three keys to successfully vernalizing a perennial and if any one factor is not satisfied, vernalization will not be successful.
1. The plant being vernalized must be mature (not juvenile).
2. Cooling must be at the right temperature (generally no higher than 38-41° F average)
3. Cooling for the right length of time (generally at least six to 10 weeks).
Juvenility is typically the most troublesome factor in vernalization. Juvenile perennials will not respond to a cold treatment, but the length of the juvenile period is not well understood for many perennials, and juvenility can vary among cultivars of the same species. The response of juvenile perennials after cooling can also vary significantly. In some species, plants will flower in low percentages, while in others species plants will not flower at all. In some perennials, vernalization is not required but may significantly reduce the time to flower after transplant or improve flowering uniformity.
Though most perennials require extended cold temperatures to successfully vernalize, there are some low vernalization requirement cultivars that can greatly simplify the vernalization process. Perennials like Bellis perennis, many primula species and a few new aquilegia series (notably Origami and Swan) will successfully vernalize when exposed to temperatures as high as 45-50° F.
Choosing a Plug or Liner
Perennial plugs and liners are available in a wide range of sizes, from small 338- or 288-cell trays; to intermediate 72-, 128- or 200-cell trays; to large liners like 50-, 32- or 21-cell trays. Growers should base the decision of which size to use on the size of the finished container, desired sales window, production goals (green vs. flowering sales), production schedule and budget.
The larger the plug or liner, the more foolproof production will generally be. Especially in the first two weeks after transplant, moisture management is critical for smaller plugs, particularly if they are transplanted into large containers. Larger plugs provide more margin for error during establishment. As growers gain confidence and experience with perennials, they may be able to move their production to smaller, more economical plugs and liners.
Small plugs are ideal for finishing packs or small containers, and multiple plugs can be planted into containers as large as gallon pots. In general, larger plugs and liners finish faster than smaller plugs and liners, and large containers take longer to finish than small containers. In gallon or larger containers, two to three plugs may be planted to help ensure a good container fill.
Small plugs also can be transplanted into packs, small containers, or large-volume plug trays and “bumped up” into larger containers later. Some growers use this strategy to do their own vernalization of large liners, carrying the transplanted trays through the winter and transplanting the vernalized liners into larger containers in the late winter or early spring.
Medium-sized plugs are especially versatile, being suitable for small or large containers. Transplanting two to three plugs when growing in large containers will result in faster-filling, fuller containers.
Vernalized liners are another option perennial growers can use to their advantage. Vernalized perennial liners are typically available only from late winter to mid-spring. The level of dormancy in vernalized liners can vary among suppliers, depending on the cooling temperatures used. In some species, top growth will die back completely during the vernalization process.
Vernalized liners are preconditioned for cool growing conditions (55-60° F or colder), but can also be grown warm (65-75° F). Roots develop quickly from vernalized liners, often faster than from standard liners. However, top growth may initially be slower with vernalized liners than standard liners. It may take two to four weeks longer for plants to fill pots with vernalized liners compared to standard liners, though finished containers are typically fuller with more breaks.
Thanks to the work of perennial breeders, growers now have numerous options for growing first-year-flowering perennials. These cultivars do not require vernalization to flower, so they can be grown more like annual crops. The “perennial” nature of a cultivar is sometimes reduced when breeders remove the vernalization requirement, but this is not always the case, and there are some good, truly perennial first-year-flowering cultivars available.
Some breeders have developed marketing programs to promote their first-year-flowering perennials. Benary’s FastraX perennials and Kieft’s Prime Perennials are examples of such programs.
Not every perennial, even if it flowers the first year, is suitable for every sales window. For spring production, focus on day-neutral perennials, which typically prefer cool growing conditions. Examples include: ajuga, aquilegia, arabis, aubrieta, bellis, Coreopsis auriculata, dianthus, doronicum, lamium, Papaver nudicaule, Phlox subulata, many types of primula and viola. Many of these spring-blooming perennials don’t perform well during the heat of summer, particularly in hot climates, resulting in poor finish quality. In addition, artificial lighting can be used to bring long-day perennials into bloom for spring sales.
Good perennials for summer sales windows flower under long days and have a reasonable level of heat tolerance. Examples include: most coreopsis species, digitalis, echinacea, gaillardia, gaura, hibiscus, lavandula, leucanthemum, monarda, nepeta, penstemon, salvia, rudbeckia and veronica.
Perennials suitable for fall sales can take advantage of cooling temperatures and shortening days. Summer-flowering perennials with long bloom periods are also suitable for fall sales. In climates with moderate summers, many first-year-flowering, spring-blooming perennials are also suitable for fall production. Examples include: Aster novi-belgii, helenium, rudbeckia, Scabiosa columbaria, solidago and viola.
Foliage Perennials (Blooming Not Needed)
There is a wide range of perennials grown for their attractive foliage over their flowers. Perennials grown for sale green will usually fill containers within four to 10 weeks after transplant, depending on the plug or liner used, container size and growing temperature. Growers can speed up finish in large containers by transplanting two to three plugs per container. Good foliage perennials include: Artemesia ‘Silver Brocade’, ‘Silver Mound’, ‘Powis Castle’; Brunnera ‘Jack Frost’; Heuchera micranthra ‘Palace Purple’; Heuchera x hybrida cultivars; Lobelia x hybrida ‘Queen Victoria’; Penstemon digitalis ‘Husker’s Red’; pulmonaria cultivars; stachys and ornamental grasses (including PanAmerican’s Fantastic Foliage grasses and Kieft’s ColorGrass varieties).